Testosterone metabolites differentially maintain adult morphology in a sexually dimorphic neuromuscular system

Abstract

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). Androgens are necessary for the development of the SNB neuromuscular system, and in adulthood, continue to influence the morphology and function of the motoneurons and their target musculature. However, estrogens are also involved in the development of the SNB system, and are capable of maintaining function in adulthood. In this experiment, we assessed the ability of testosterone metabolites, estrogens and nonaromatizable androgens, to maintain neuromuscular morphology in adulthood. Motoneuron and muscle morphology was assessed in adult normal males, sham-castrated males, castrated males treated with testosterone, dihydrotestosterone, estradiol, or left untreated, and gonadally intact males treated with the 5alpha-reductase inhibitor finasteride or the aromatase inhibitor fadrozole. After 6 weeks of treatment, SNB motoneurons were retrogradely labeled with cholera toxin-HRP and reconstructed in three dimensions. Castration resulted in reductions in SNB target muscle size, soma size, and dendritic morphology. Testosterone treatment after castration maintained SNB soma size, dendritic morphology, and elevated target muscle size; dihydrotestosterone treatment also maintained SNB dendritic length, but was less effective than testosterone in maintaining both SNB soma size and target muscle weight. Treatment of intact males with finasteride or fadrozole did not alter the morphology of SNB motoneurons or their target muscles. In contrast, estradiol treatment was completely ineffective in preventing castration-induced atrophy of the SNB neuromuscular system. Together, these results suggest that the maintenance of adult motoneuron or muscle morphology is strictly mediated by androgens.

Figure 1

Copulatory behavior after exposure to sexually receptive females in gonadally intact and fadrozole-treated males, as assessed by perivaginal inspection (top, left), latency to first mount (top, right), number of mounts (bottom, left), and inter-mount latency (bottom, right). Bar heights represent means ± SEM. *Significantly different from normal males.

Figure 2

Weights of BC/LA muscles expressed as raw weights (top) and relative to individual body weights (bottom) of normal, sham-castrated, castrated, testosterone (T)-treated castrated, dihydrotestosterone (DHT)-treated castrated, finasteride (FIN)-treated, estradiol (E)-treated, or fadrozole (FAD)-treated males. Bar heights represent means ± SEM. *Significantly different from normal males.

Figure 3

(left) Dark-field photomicrographs of transverse sections through the lumbar spinal cord of a normal (top), castrated (middle), and testosterone (T)-treated castrated male after BHRP injection into the left BC muscle. Scale bar = 500 μm. (right) Computer-generated composites of BHRP-labeled SNB somata and processes drawn at 320 μm intervals through the entire rostrocaudal extent of the SNB; these composites were selected as they are representative of their respective group average dendritic lengths.

Figure 4

(left) Dark-field photomicrographs of transverse sections through the lumbar spinal cord of a dihydrotestosterone (DHT)-treated castrated (top), finasteride (FIN)-treated (middle, upper), estradiol (E)-treated castrated (middle, lower) and fadrozole (FAD)-treated male after BHRP injection into the left BC muscle. Scale bar = 500 μm. (right) Computer-generated composites of BHRP-labeled SNB somata and processes, drawn and selected as in Fig. 3.

Figure 5

Soma areas of SNB motoneurons in normal, sham-castrated, castrated, testosterone (T)-treated castrated, dihydrotestosterone (DHT)-treated castrated, finasteride (FIN)-treated, estradiol (E)-treated, or fadrozole (FAD)-treated males. Bar heights represent means ± SEM. *Significantly different from normal males.

Figure 6

Dendritic lengths expressed as length of arbor per labeled motoneuron in normal, sham-castrated, castrated, testosterone (T)-treated castrated, dihydrotestosterone (DHT)-treated castrated, finasteride (FIN)-treated, estradiol (E)-treated, or fadrozole (FAD)-treated males. Bar heights represent means ± SEM *Significantly different from normal males.

Figure 7

(inset) Schematic drawing of spinal gray matter divided into radial sectors for measure of SNB dendritic distribution. Length per radial bin of SNB dendrites in normal, sham-castrated, castrated, testosterone (T)-treated castrated, dihydrotestosterone (DHT)-treated castrated, finasteride (FIN)-treated, estradiol (E)-treated, or fadrozole (FAD)-treated males; for graphical purposes length per radial bin measures have been collapsed into 6 bins of 60° each. Bar heights represent means ± SEM. *Significantly different from normal males.

Figure 8

(inset) Schematic drawing of spinal gray matter divided into radial sectors for measure of SNB radial extent. Radial extents of SNB dendrites in normal, sham-castrated, castrated, testosterone (T)-treated castrated, dihydrotestosterone (DHT)-treated castrated, finasteride (FIN)-treated, estradiol (E)-treated, or fadrozole (FAD)-treated males; for graphical purposes dendritic extent measures have been collapsed into 6 bins of 60° each. Bar heights represent means ± SEM.